Flow-through hollow cathode spectral light source and method of operating same

ABSTRACT

An atomic spectral lamp includes a demountable, hollow cathode, and a funnel-shaped anode adjacent to the cathode. A glow discharge plasma is established in the cathode cavity. Counterflow through the funnel-shaped anode prevents self absorption of light by atomic vapor. Gas flow through the hollow cathode at a predetermined velocity increases the intensity of light-emitted from the plasma.

United States Patent Chaney 1 Oct. 17, 1972 [54] FLOW-THROUGH HOLLOWCATHODE 2,779,890 1/1957 Frenkel ..313/209 X SPECTRAL LIGHT SQURCE AND3,543,077 ll/ l970 Grimm ..356/86 X METHOD OF OPERATING SAME, FOREIGNPATENTS OR APPLICATIONS [m Chaney San 1,244,956 7/1967 Germany ..313/209I73] Assigncc: Hewlett-Packard Company, Palo Alto, Calif. PrimaryExaminer-Palmer C. DeMeo I221 Filed' Dec 28 1970 [21] A II No.: 101,8721- B R An atomic spectral lamp includes a demountable, hol- I p lowcathode, and a funnel-shaped anode adjacent to [52] CL ag 32 3 thecathode. A glow discharge Plasma is established in 1 r the cathodecavity. Counterflow through the funnel- [51 Int. Cl .1101] 6 I06, H01] 6/28l Shaped ano prevents self absorption of light y [58]. held M Search"313/209! atomic vapor. Gas flow through the hollow cathode at 356/85,86, 8 315/l a predetermined velocity increases the intensity oflight-emitted from the plasma. [56] References Cited UNITED STATESPATENTS 12 Claims, 6 Drawing Figures 3,401,292 9/19 8 Cirri ..313/209 x63 II e1 Y 12 ('73 57 53 3 29 FLOW 59 *La'alt. 37 REGULAT0R I 2* n u 1/3 67 2; 47 o 41 43 POWER l' SUPPLY VACUUM PUMP :l

PATENTEDUBT n 1912,

SHEET 1 BF 2 igure 1 FLOW GULATOR POWER SUPPLY Ju re 2 VACUUM PUMPINVENTOR ROBERT L. CHANEY BY M Z:

ATTORNEY igure 3 RELATIVE SIGNAL PATENTEDum 17 I972 SHEET 2 OF 2 2-8 I I'l l I ll I I 877 91 2.4- 9

RELAnvE meNAL 1 G 2 l l l I I CATHODE FLOW VELOCITY x1'o cm/sec RELATIVESIGNAL POWER INPUT Wofls Fi ure 4b 15 Watts 1O Waits A 20 Watts I Q i t1 2 3 4 5 s PRESSURE T igure 4a INVENTOR ROBERT L. CHANEY BY 12 figATTORNEY FLOW-THROUGH HOLLOW CATHODE SPECTRAL LIGHT SOURCE AND METHOD OFOPERATING SAME i BACKGROUND OF THE INVENTION Hollow cathode lamps havefound widespread use as sources of high intensity Spectral lines.Typically, such a. lamp includes a hollow cathode element in anevacuated sealed housing. The cathode is formed of the particularmaterial which provides the spectral lines desired. A d.c. or r.f. glowdischarge plasma is used to sputter the cathode material to produce freeatoms.

These atoms are then excited by the plasma to produce spectral emission.

Recent advances have been made in improving the adaptability andefficiency of hollow cathode lamps. For example, cathode elements havebeen made demountable to permit cathodes of different materials, andthus different spectral line outputs, to be inserted in the lamp. Also,a flow of inert gas has been provided through the lamp in a directioncounter to that of the emitted light. This gas counterflow reduces theconcentration of atoms between the light emitting cathode and the lightexit window at high discharge currents, which in turn prevents theself-absorption of emitted light. As a result, the intensity ofradiation emitted by the lamp is increased and spectral line widths arenarrowed.

Continuing efforts have been made to increase the light intensityand'narrow the line widths of spectral SUMMARY OF THE INVENTION Thepresent invention is a hollow cathode spectral ing the intensity of thespectral line output over that of conventional hollow cathode sources.The illustrated embodiment of the invention includes a demountablehollow cathode element disposed in a vacuum chamber in optical alignmentwith a light exit window in the chamber. A funnel-shaped anodepositioned between the hollow cathode and the window has a narrow openend which terminates in spaced-apart relationship with the cathode. Apotential difference applied between the cathode and anode establishes aglow discharge plasma in the open-ended cavity of the hollow cathode. Acounterflow of inert gas is directed through the funnel-shaped anodetoward the cathode to prevent selfabsorption of light by free atomsdiffusing toward the window.

A feature of the invention is that the hollow cathode element includes agas passageway coupled to a gas inlet conduit. The gas passagewaydirects an inert gas into the cathode cavity from its closed end and outthrough its open end. The gas flow velocity through the cathode cavityand the vacuum chamber pressure are set to selected values, thereby topermit optimization of the intensity of light emitted from the glowdischarge plasma. The intensity of light produced is on the order of tentimes greater than that obtained from the most I light sourceincorporating a method for greatly increas- 2 t efficient heretoforeknown hollow cathode light sources, and to 1,000 times greater than.that produced by conventional sealed glass lamps.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an external perspective viewof the hollow cathode lamp incorporating the present invention.

FIG. 2 is a combined block diagram and longitudinal cross-sectional viewof the lamp shown in FIG. 1.

FIG. 3 is an enlarged cross-sectional view of the demountable hollowcathode element shown in FIG. 2.

FIGS. 4a-c are graphs illustrating various operating characteristics ofthe hollow cathode lamp incorporating the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT member for rigidly holding thelamp in position. A light transparent window or lens 19 is fixed insealed relation to the housing 1 1 and held in place by a retainer ring21 to form one end of the vacuum chamber. Light emitted by the lamp istransmitted through the window 19 in the direction'of the arrow23. Twogas inlet conduits 25 and 27 direct an inert gas into the vacuum chamberinternal of the housing 11, for purposes hereinafter described.

FIG. 2 shows a vertical, longitudinal cross-sectional view of the lampillustrating the internal configuration thereof. A cylindrical hollowcathode element 29 is disposed in the vacuum chamber 31 formed by thetubular housing 11. The cavity 33 in the hollow cathode 29 has an openside facing the window 19. As describedabove, the window 19 is securedby the retainer ring 21. This ring is made of plastic and has a threadedrim portion which screw-mounts onto the end of the housing 11. A vacuumtight seal is maintained between the window 19 and the housing 11 by'arubber O-ring 35. Disposed between the cathode element 29 and the window19 is a cylindrical anode element 37, constructed of stainless steel,and fitted inside the cylindrical vacuum chamber. The cylindrical anodeelement 37 has a funnel-shaped internal bore. The wide end of the bore39 is disposed in spaced-apart relation to the window l9 and the narrowend of the bore 41 is disposed in spaced-apart relation to the cathodeelement 29.

The cylindrical anode element 37 is configured so that the externaldiameter thereof at the end nearest to the window 19 is slightly smallerthan the internal diameter of the vacuum chamber 31, thereby to form anannular open passageway in communication with the wide end of thefunnel-shaped bore in the anode electrode. A gas inlet port 43 in theside wall of housing 11 is coupled to the annular gas passageway. Asdescribed hereinafter, an inert gas is fed into the port 43, through theannular passageway, and into the open end of the funnel to produce a gascounterflow through the vacuum chamber and out through the vacuum portand pipe 13 to the vacuum pump 44.

The hollow cathode element 29 is shown in more detail in FIG. 3.Referring to FIG. 3 in conjunction with FIG. 2, it can be seen that thecathode element has a threaded end portion 45 which is screw-mounted onthe internally threaded end of a copper tube 47. This tube extendsthrough an aperture in the end wall of housing 11, and the left end ofthe tube (as viewed in FIG. 2) terminates in a flanged portion 49.Surrounding the copper tube 47 is another tube 51 which also has aflanged end portion disposed contiguously with the flange 49. The tube51 is formed of a suitable insulating material, such as alumina. Arubber washer 53 surrounds the flange of the insulating tube 51 andabuts one side of the flanged portion 49. Abutting the other side offlange 49 is a hollow tubular insulating member 55, formed of alumina,for example. Another stainless steel tube 59 having a flanged endportion 57 abuts the other end of the insulating member 55. Member 55and the flanged portion 57 are set into a bore in a plastic retainingcap 61. As shown, the retaining cap 61 also includes a smaller bore atthe left hand end thereof for receiving the stainless steel tube 59. Thecap 61 includes a threaded rim portion 63 which is screwmounted on theend of the cylindrical housing 11.

The above-described cathode mounting structure is assembled by firstscrewing the cathode element 29 into the end of the copper tube 47 thensliding the insulating tube 51 over the tube 47 and positioning therubber washer 53 around the flange of tube 51. This assembly is theninserted into the vacuum chamber 31 through the bore at the end ofhousing 11 into the position shown in FIG. 2. Thereafter, the plasticcap 61 containing the flange 57 and insulating tube 55 is screwed ontothe end portion of the housing 11 and securely tightened. Thisarrangement securely mounts the hollow cathode inside the vacuum chamberin a manner such that the hollow cathode element 29 may easily bedemounted, i.e., removed from the lamp. As a result, the cathode elementmay easily be replaced with another cathode element of a differentmaterial when a different spectral line source is desired. The cathodemounting assembly also provides a conduit so that an inert gas may bedirected through the inlet pipe 59 to the cathode element 29. Thejunction points along the components forming the gas conduit aresuitably sealed from the external environment by O-rings 65 and 67. Thepurpose of the gas flow through this conduit is described hereinafter.

The cathode element 29 is electrically coupled through the copper tube47 and flange 49 to the negative output terminal of a power supply 69.The positive output of the power supply is grounded and coupled to thehousing 11. The narrow opening 41 of the funnel in the anode electrode37 is in electrical contact with the housing 11 and thus is maintainedat ground potential. Power supply 69 establishes a potential differenceof 150 to 350 volts between the hollow cathode element 29 and theadjacent portion 41 of the anode element. The particular voltage dependson the power input to the lamp during operation. The cathode anodepotential difference causes a glow discharge plasma to be formed. Theplasma ismade up of free electrons and ions of the inert gas in thevacuum chamber. The plasma causes sputtering of atoms from the cathodeelement. These atoms are excited by the glow discharge and as a result,they provide spectral line radiation. The radiation is directedoutwardly through the lens or exit window 19. Due to the configurationand spacing of the cathode and anode elements, the plasma does not enterthe funnel in the anode.

Atoms which have been sputtered from the cathode tend to diffuse towardthe exit window and form a cloud of atomic vapor between the cathode andthe window. The atomic vapor cloud is undesirable because it absorbsemitted light. The gas counterflow from the wide end to the narrow end,of the funnel in the anode element 37 disperses or sweeps away atomswhich would otherwise form a vapor cloud between the cathode and thewindow. As a result, self-absorption of emitted radiation by free atoms.is eliminated and the intensity of the light output is increased.

As shown most clearly in FIG. 3, the hollow cathode element 29 includesan axial bore 71 therethrough which acts as a gas passageway to theinternal left hand end portion of the cavity 33. This gas passageway isin gas communication with the above-described conduit means formed bythe copper tube 47, the tubular insulator 55 and the stainless steeltube 59. An inert gas is directed through a flow regulator 73 and thencethrough the conduit means and the gas passageway 71 in the cathodeelement. The gas enters at one end of the cavity 33, and passes throughit to emerge at the open end of the cavity. A significant feature ofthis invention is that the flow of gas through the cathode greatlyincreases the intensity of radiation emitted from the glow dischargeplasma in the cavity 33, as described hereinafter.

As shown, the gas passageway 71 has a cross-sectional area substantiallysmaller than that of the adjacent upstream portion of the tube 47 onwhich the cathode element is mounted. This difference in crosssectionalareas serves to create a higher gas pressure in the tube 47 than in thegas passageway 71. The decreasing gas pressure differential along theflow path prevents the glow discharge plasma from moving upstream intothe tube 47. Thus, the entire discharge plasma region is maintained inoptical alignment with the window 19.

The inert gas directed through the gas passageway 71 may be selectedfrom the group of helium, neon, argon, krypton and xenon. The same inertgas may be used in the gas counterflow through anode 37.

The operation of the flow through hollow cathode lamp can best beunderstood with reference to the curves illustrated in FIGS. 4ac. Thesecharacteristic curves were obtained using a hollow cathode element asshown in FIG. 3 and having a cylindrical gas passageway 71 of diameterD, equal to 0.013 inch, a cylindrical cavity of an internal diameter Dequal to 0.075 inch, and an internal cavity length L of 0.150 inch. FIG.4a shows the relative signal output, i.e., the intensity of lightemitted by the lamp, as a function of the pressure in the vacuum chamber31 for selected magnitudes of the input power applied to the glowdischarge plasma by the power supply 69. The lower three curves in thefamily of curves 75 correspond to light outputs measured with no gasflow through the cathode element 29; whereas the upper three curvesforming the family of curves 77 correspond to light outputs when thereis a gas flow through the gas passageway 71 of the cathode element at aselected velocity of 6.3 X centimeters per second. This flow velocity isachieved when the diameter D, of the gas passageway 71 is 0.013 inch indiameter and the gas flow into it is at the rate of 9 cubic centimetersper minute. It can be seen that for the particular cathode configurationdescribed above, the optimum operating pressure in the vacuum chamber isabout 4 to 5 Torr (as measured at the exhaust port 13). At thispressure, the light output from the lamp with the gas flow through thecathode is about ten times the light output with no flow through thecathode. It should also be noted that both families of curves 75, 77were derived timeters per minute. Conventional sealed glass hollowcathode lamps do not have this counterflow feature and the lightoutputtherefrom is much lower. The output signals from such lamps arenot shown in FIG. 4a because they are on the order of 10 to 100 timesless than the signals represented by curves 75. Thus, the signal fromthe lamp having a gas flow through the cathode (curves 77) compared tothat from conventional sealed glass hollow cathode lamps is on the orderof 100 to l,000 times greater.

FIG. 4b illustrates the relative signal output from the lamp as afunction of the power input thereto from the power supply 69 fordifferent pressures in the vacuum chamber 31. The family of curves 79represent the light output with a gas flow velocity through the cathodepassageway 71 of 6.3 X 10 centimeters per second, as described above;whereas the curve 81 represents light output with no gas flow throughthe cathode. Only one curve 81 is shown for the no cathode flow casebecause there is no substantial difference in light output for differentvalues of pressure in the vacuum chamber in the range of 2 to 5 Torr. Itcan be seen in FIG. 4b, as in FIG. 4a, that relative signal outputincreases with the power input, and that the optimum pressure in thevacuum chamber is about 4 Torr. Also, as shown in FIG. 4b, lightradiated in the presence of a gas flow through the cathode is on theorder of 10 times greater than a lamp using only a gas counterflow, andthus 100 to 1,000 times better than the conventional sealed glass hollowcathode lamps without a gas counterflow.

FIG. 4c illustrates the signal output of the lamp as a function of thegas flow velocity through the gas passageway 71 of the hollow cathodeelement. The two portions of the curve 83, 85 represent an unstableregion of operation of the lamp at cathode flow velocities below about5.5 X 10" centimeters per second. In this region, the lamp operation mayflip back and forth between the two curves 83, 85, thus producing eithera high or low intensity light output. Also, in the two portions of thecurve 87, 89 represented by the dashed lines, the lamp operation isunstable because the glow discharge plasma oscillates. It has been foundthat there are two regions of stable lamp operation with the cathodeconfiguration described above. These two stable regions are in theportions of the curve 91, 93 and correspond to gas flow velocities inthe range of 6.0 to 6.5 X 10 centimeters per second (region 91) andhigher velocities in the range of 8.5 to 10 X 10 centimeters per second.(region 93). It is to be noted that the characteristic curves of FIGS.4a and 4b were obtained using a gas flow velocity of 6.3 X 10centimeters per second, which is within the region 91 shown in FIG. 40.

In summary, it can be seen that the provision of gas flow through thehollow cathode greatly increases the light output from the hollowcathode lamp. The radiation emitted by the glow discharge plasma isoptimized by setting the gas flow velocity and the vacuum chamberpressure to selected values.

I claim:

I I 1. An atomic spectral light source comprising:

a vacuum chamber having a window portion and a vacuum port,

a hollow cathode element' having an opencavity therein disposed in saidchamber in optical alignment with said window portion, said cathodeelement also having a gas passageway therethrough communicating with aninternal portion of said cavity; means for producing a glow dischargeplasma in said cavity; conduit means coupled to the gas passageway insaid cathode element and having a gas inlet for directing a gas flowthrough the cavity of said cathode element at a predetermined velocity,thereby to permit optimization of the intensity of light emitted by saidglow discharge plasma in said caviy;

a member disposed between said cathode element and the window portion ofsaid vacuum chamber, said member having an internal bore configured inthe shape of a funnel having a narrow opening adjacent to said cathodeelement and a wide opening adjacent to said window portion; and I gasinlet and conduit means for directing a flow of inert gas into. the wideopening of said funnel adjacent to said window portion, thereby to causegas flow through said funnel in a direction opposite to that of theradiation out of said cavity toward said window portion.

2. The apparatus of claim 1 wherein the cross-sectional area of said gaspassageway in said cavity member is less than that of said conduit meansto cause a decreasing gas pressure differential from said conduit meansto said gas passageway, thereby to prevent formation of a glow dischargeplasma in said conduit means.

3. The apparatus of claim 1, further including means for controlling theflow velocity of gas which is passed through said passageway into saidcavity.

4. The apparatus of claim 1, wherein the gas flow through said gaspassageway is at the velocity of at least 6.0 X 10 centimeters persecond.

'5. The apparatus of claim 4, wherein the gas flow velocity through saidgas passageway is in one of the ranges of 6.0 to 6.5 X 10 and 8.5 to 10X 10 centimeters per second.

6. The apparatus of claim 4, wherein the gas directed through saidcavity is an inert gas selected from the group consisting of helium,neon, argon, krypton and xenon.

7. The apparatus of claim 1, wherein said means for producing a glowdischarge plasma in said cavity includes:

an anode element in said vacuum chamber disposed in spaced-apartrelation with said cathode element; and

means for applying a positive potential to said anode element and anegative potential to said cathode element.

8. The apparatus of claim 1, wherein said member serves as an anodeelement and said means for producing a glow discharge plasma in saidcavity includes means for applying a positive potential to said memberand a negative potential to said cathode element.

9. In an atomic spectral light source including the combination of avacuum chamber having a window portion and-a vacuum port; a hollowcathode element disposed in said chamber so that the open portion of thecavity in said element is optically aligned with said window portion; amember disposed between said cathode element and said window portionhaving an in ternal bore configured in the shape of a funnel having anarrow opening adjacent to said cathode element and a wide openingadjacent to said window portion; and means for producing a glowdischarge plasma in said cavity; a method for increasing the intensityof light emitted from said glow discharge plasma comprising:

passing an inert gas through a gas passageway in said hollow cathodeelement into an internal portion of said cavity and out through the openportion of said cavity at a predetermined velocity of flow through saidpassageway -while maintaining a predetermined pressure in said vacuumchamber, and directing a flow of inert gas into the wide opening of saidfunnel member, thereby to cause gas flow through said funnel in adirection opposite to that of the radiation out of said cavity towardsaid window portion. 10. The method of claim 9, wherein said flowvelocity in said gas passageway is at least 6.0 X 10 centimeters persecond.

11. The method of claim 10, wherein said flow velocity is in one of theranges of 6.0 to 6.5 X 10 and 8.5 to 10.0 X 10 centimeters per second.

12. The method of claim 9, wherein said inert gas is selected from thegroup consisting of helium, neon, argon, krypton and xenon.

1. An atomic spectral light source comprising: a vacuum chamber having awindow portion and a vacuum port; a hollow cathode element having anopen cavity therein disposed in said chamber in optical alignment withsaid window portion, said cathode element also having a gas passagewaytherethrough communicating with an internal portion of said cavity;means for producing a glow discharge plasma in said cavity; conduitmeans coupled to the gas passageway in said cathode element and having agas inlet for directing a gas flow through the cavity of said cathodeelement at a predetermined velocity, thereby to permit optimization ofthe intensity of light emitted by said glow discharge plasma in saidcavity; a member disposed between said cathode element and the windowportion of said vacuum chamber, said member having an internal boreconfigured in the shape of a funnel having a narrow opening adjacent tosaid cathode element and a wide opening adjacent to said window portion;and gas inlet and conduit means for directing a flow of inert gas intothe wide opening of said funnel adjacent to said window portion, therebyto cause gas flow through said funnel in a direction opposite to that ofthe radiation out of said cavity toward said window portion.
 2. Theapparatus of claim 1 wherein the cross-sectional area of said gaspassageway in said cavity member is less than that of said conduit meansto cause a decreasing gas pressure differential from said conduit meansto said gas passageway, thereby to prevent formation of a glow dischargeplasma in said conduit means.
 3. The apparatus of claim 1, furtherincluding means for controlling the flow velocity of gas which is passedthroUgh said passageway into said cavity.
 4. The apparatus of claim 1,wherein the gas flow through said gas passageway is at the velocity ofat least 6.0 X 105 centimeters per second.
 5. The apparatus of claim 4,wherein the gas flow velocity through said gas passageway is in one ofthe ranges of 6.0 to 6.5 X 105 and 8.5 to 10 X 105 centimeters persecond.
 6. The apparatus of claim 4, wherein the gas directed throughsaid cavity is an inert gas selected from the group consisting ofhelium, neon, argon, krypton and xenon.
 7. The apparatus of claim 1,wherein said means for producing a glow discharge plasma in said cavityincludes: an anode element in said vacuum chamber disposed inspaced-apart relation with said cathode element; and means for applyinga positive potential to said anode element and a negative potential tosaid cathode element.
 8. The apparatus of claim 1, wherein said memberserves as an anode element and said means for producing a glow dischargeplasma in said cavity includes means for applying a positive potentialto said member and a negative potential to said cathode element.
 9. Inan atomic spectral light source including the combination of a vacuumchamber having a window portion and a vacuum port; a hollow cathodeelement disposed in said chamber so that the open portion of the cavityin said element is optically aligned with said window portion; a memberdisposed between said cathode element and said window portion having aninternal bore configured in the shape of a funnel having a narrowopening adjacent to said cathode element and a wide opening adjacent tosaid window portion; and means for producing a glow discharge plasma insaid cavity; a method for increasing the intensity of light emitted fromsaid glow discharge plasma comprising: passing an inert gas through agas passageway in said hollow cathode element into an internal portionof said cavity and out through the open portion of said cavity at apredetermined velocity of flow through said passageway while maintaininga predetermined pressure in said vacuum chamber, and directing a flow ofinert gas into the wide opening of said funnel member, thereby to causegas flow through said funnel in a direction opposite to that of theradiation out of said cavity toward said window portion.
 10. The methodof claim 9, wherein said flow velocity in said gas passageway is atleast 6.0 X 105 centimeters per second.
 11. The method of claim 10,wherein said flow velocity is in one of the ranges of 6.0 to 6.5 X 105and 8.5 to 10.0 X 105 centimeters per second.
 12. The method of claim 9,wherein said inert gas is selected from the group consisting of helium,neon, argon, krypton and xenon.